1. Field of the Invention
This invention relates to hydraulic fluid compositions, particularly brake fluid compositions for use in hydraulic brake systems of automobiles.
2. Description of the Prior Art
Recently, automobiles have tended to become faster and larger, at the same time that greater safety is required. For this purpose, hydraulic fluids of higher performance are strongly demanded.
The first requirement for hydraulic fluids is to be free from the so-called "vapor lock phenomenon". This phenomenon is caused by the vaporization of hydraulic fluids and makes brake control impossible. Consequently, brake fluids having a higher boiling point are demanded. Efforts have been made to develop hydraulic fluids having a high boiling point even in the moist state and which can maintain the higher boiling point for a long period of time. the conventional hydraulic fluids which contain a high molecular weight polyether as base polymer and a low molecular weight glycol ether as diluent, are hygroscopic and tend to suffer a severe drop in their boiling points attendant upon moisture absorption. Such hydraulic fluids are therefore unlikely to pass the standard of DOT 4 (higher than 155.degree. C) with respect to the wet equilibrium reflux boiling point (boiling point in a moist state), according to the hydraulic fluid specification of U.S. Department of Transportation [DOT]). Hitherto, there have been proposed several hydraulic fluids which contain borate esters of glycol ethers. Such hydraulic fluids may be adequate regarding their wet equilibrium reflux boiling points, but are not effective in inhibiting the inherent hydrolyzability of the borate esters. A need exists therefore, for hydraulic fluids having a high resistance to hydrolysis and at the same time a high enough wet equilibrium reflux boiling point to pass the standard of DOT 4.